Alas, this is not the case, at least based on the 136.avi file linked to at the bottom of DC.Watch’s D300S review.
Here’s an example from my D5000 (click to view a larger version of the screen capture). Notice that the data rate is at 22.41 mbits/s. Audio is at 11.025 kHz.
The next screen capture relates to the D300S footage. The data rate is 22.86 mbits/s, and the audio is at 11.025 kHz.
Looking at the comparisons, and based on my own viewing and listening, it does look like Nikon made no improvements in this area.
ArmatureMike does report, however, that there is less rolling shutter / jello / wobble effects in the D300S movies, so this is at least one piece of good news.